The present invention relates to a self-driving circuit for a DC/DC converter of low voltage, high current, and high power density.
With the rapid development of high technologies, such as, communications, remote sensing, electronic computers, and electronic instrument, the requirement of power supplies of such electronic equipment has increased accordingly. DC/DC converter of low voltage, high current, and high power density is the core technology of the power supplies for supper integrated circuits and high-speed central processors. To meet high efficiency and high power density requirement, the auxiliary (secondary) side of such DC/DC converters shall use synchronous rectifying MOSFET transistor in place of Schottky diode for rectification in order to reduce power loss. However, for a synchronous rectifying MOSFET, the gate thereof needs a corresponding drive circuit to stimulate. In order to prevent cross-conductive losses, the requirement of time sequence of the drive circuit is very high. The existing drive circuits utilize external driving technology, but its control is too complicated and the cost is high.
For a converter having only one synchronous rectifying MOSFET at the secondary side, such as a backward stimulating circuit, the synchronous rectifying MOSFET cannot be driven directly by the waveform of the secondary side of the transformer. Otherwise, the transistor would be damaged for failure to shut down. For instance, FIG. 1a shows a traditional backward stimulating circuit. The voltage waveform of its secondary side is shown in FIG. 1b. If such a waveform of the secondary side is used to drive the gate of SR1, as shown in FIG. 2, SR1 may hardly be turned off owing to Vgs1=Vo, and there will be short-circuit in the secondary side to cause losses when S turns on.
If an external auxiliary winding is used for self-driving, the converting efficiency may reduce dramatically because it is hard to control the cross-conducting time. The efficiency is normally lower than that of using Schottky diode. For instance, FIG. 3 shows a self-driving circuit using an external auxiliary winding Nsa to drive SR1. When S turns off and the secondary side voltage changes to positive at top and negative at bottom, SR1 turns on, and the energy of the secondary side of the transformer will be provided to the load through SR1; when S turns on, S and SR1 may be on at the same time because it will need sometime for the secondary side voltage to change to negative at bottom and positive at top such that the secondary side would be short-circuited. Although this time is relatively short, its cross-conductive loss is rather high. If serious, it may damage S and SR1. Even under the normal operation, the converting efficiency can hardly be increased. Therefore, to improve the converting efficiency, the self-driving circuit should be modified.
Therefore, the object of the present invention is to solve the existing problem in the self-driving circuit of the main circuit of a DC/DC converter of low voltage and high current, and to provide a self-driving circuit for the converter that has reduced cross-conductive loss, simple structure, and low cost.
The present invention is realized through the following technical embodiments. In the first configuration of the self-driving circuit of the DC/DC converter of the present invention, the rectification portion of the converter comprises a synchronous rectifying MOS transistor (SR1), wherein the self-driving circuit is composed of two resisters (Ra1, Ra2), two capacitors (Ca1, Ca2), a PNP transistor (Qa1) and a NPN transistor (Qa2). The resister (Ra1) and the capacitor (Ca1) are connected in parallel, and an end of the parallel connection of the register (Ra2) and capacitor (Ca2) connected with the base of the transistor (Qa) and the other end connected with an end of parallel connected resister (Ra2) and capacitor (Ca2), and with the positive end of the transformer winding (Ns) and the drain end of the rectifying MOS transistor (SR1). The other end of the parallel connection of the resister (Ra2) and capacitor (Ca2) is connected with the base end of the transistor (Qa2). The emitter of the transistor (Qa2) is connected with the source end of the MOS transistor (SR1), while its collector is connected the collector of the transistor (Qa1) and the gate of the MOS transistor (SR1). The emitter of the transistor (Qa1) is connected with the negative end of the winding (Ns).
In the second configuration of the self-driving circuit of the DC/DC converter of the present invention, the rectification portion of the converter comprises a synchronous rectifying MOS transistor (SR1), wherein the self-driving circuit is composed of a diode (Da), a small power MOS transistor (SRa), an auxiliary winding (Nsa), a time delay driving circuit, and an isolating differential circuit. The delay driving circuit and the isolating differential circuit are connected with each other. An end of the isolating differential circuit is connected with the gate of the small power MOS transistor SRa. The positive end of the auxiliary winding (Nsa) is connected with the source end of the small power MOS transistor (SRa) and the source end of the synchronous rectifying MOS transistor (SR1), while its negative end is connected with the anode of the diode Da. The cathode of the diode Da is connected with the gate of the synchronous rectifying MOS transistor (SR1) and the drain end of the small power transistor (SRa).
The isolating differential circuit may be composed of the windings (Npa1) and (Npa2) of the transformer, two capacitors, two resisters, and a diode. The winding Nsa1 is connected, through the capacitor, with the parallel-connected resister and diode.
The time delay driving circuit is composed of a delay circuit and a driving circuit, wherein an example of the delay circuit is formed by connecting the diode and resister in parallel, and then connected in serial to a ground capacitor.
The DC/DC converter is a double backward converter including windings (Np, Ns) and power MOS transistors (S1, S2). The positive end of the winding (Np) is connected with the source end of the power MOS transistor (S1), and the negative end of the winding (Np) is connected to the drain end of the power MOS transistor (S2). The delay driving circuit is connected with the gates of the power MOS transistors (S1) and (S2), respectively.
The DC/DC converter is a clamping backward converter of three windings (Nc, Np, Ns), including the windings (Nc, Np, Ns), power MOS transistor (S) and diode (Dc). The negative end of the winding (Np) is connected with the drain end of the power MOS transistor (S), and the positive end of the winding (Nc) is connected with the cathode of the diode (Dc). The delay driving circuit is connected with the gate of the power MOS transistor (S).
The DC/DC converter is a R.C.D. clamping backward converter including windings (Np, Ns), a power MOS transistor (S), a resister (Rc), a diode (Dc) and a capacitor (Cc). The negative end of the winding (Np) is connected with the drain end of the power MOS transistor (S). The delay driving circuit is connected with the gate of the power MOS transistor (S), and the negative end of the winding Np is connected with the anode of the diode (Dc). An end of the parallel-connected capacitor (Cc) and resister (Rc) is connected with the cathode of the diode (Dc), while the other end is connected with the positive end of the winding (Np).
The DC/DC converter is an active clamping backward converter including windings (Np, Ns), power MOS transistors (S, Sc) and a capacitor (Cc). The positive end of the winding (Np) is connected through the capacitor (Cc) with the drain end of the power MOS transistor (Sc). The source end of the power MOS transistor (Sc) is connected with the drain end of the power MOS transistor (S) and the negative end of the winding (Np). The delay driving circuit is connected with the gate of the power MOS transistor (S).
The converter is a diode clamping double backward converter including windings (Np, Ns), power MOS transistors (S1, S2) and diodes (D1, D2). The positive end of the winding (Np) is connected with the source end of the power MOS transistor (S1), and the negative end of the winding (Np) is connected with the drain end of the power MOS transistor (S2). The anode of the diode (D1) is connected with the negative end of the winding (Np), and the cathode is connected with the drain end of the power MOS transistor (S1). The anode of the diode (D2) is connected with the source end of the power MOS transistor (S2), and the cathode is connected with the positive end of the winding (Np). The delay driving circuit is connected, respectively, with the gates of the power MOS transistors (S1), (S2).
The converter is an active clamping double backward converter including windings (Np, Ns), power MOS transistors (S1, S2), a capacitor (Cc) and a power MOS transistor (Sc). The positive end of the winding (Np) is connected with the source of the power MOS transistor S1), and the negative end of the winding (Np) is connected with the drain end of the power MOS transistor (S2). The capacitor (Cc) and the power MOS transistor (Sc) are connected in serial, and then connected parallel the winding (Np), with its two ends connected with positive and negative ends of the winding (Np), respectively. The delay driving circuit is connected with the gates of the power MOS transistors (S1) and (S2), respectively.
The present invention utilizes certain small power resistance and capacitance elements, diodes, transistors or field effect transistors to realize the equivalent self-driving technology. Thus, it ensures the reliable turn-on and turn-off of SR1, and at the same time it ensures the minimum cross-conductive loss and high converting efficiency.